The collection and sampling of underground fluids contained in subsurface formations is well known. In the petroleum exploration and recovery industries, for example, samples of formation fluids are collected and analyzed for various purposes, such as to determine the existence, composition and producibility of subsurface hydrocarbon fluid reservoirs. This aspect of the exploration and recovery process can be crucial in developing drilling strategies and impacts significant financial expenditures and savings.
To conduct valid fluid analysis, the fluid obtained from the subsurface formation should possess sufficient purity, or be virgin fluid, to adequately represent the fluid contained in the formation. As used herein, and in the other sections of this patent, the terms “virgin fluid”, “acceptable virgin fluid” and variations thereof mean subsurface fluid that is pure, pristine, connate, uncontaminated or otherwise considered in the fluid sampling and analysis field to be sufficiently or acceptably representative of a given formation for valid hydrocarbon sampling and/or evaluation.
Various challenges may arise in the process of obtaining virgin fluid from subsurface formations. Again with reference to the petroleum-related industries, for example, the earth around the borehole from which fluid samples are sought typically contains contaminates, such as filtrate from the mud utilized in drilling the borehole. This material often contaminates the virgin fluid as it passes through the borehole, resulting in fluid that is generally unacceptable for hydrocarbon fluid sampling and/or evaluation. Such fluid is referred to herein as “contaminated fluid.” Because fluid is sampled through the borehole, mudcake, cement and/or other layers, it is difficult to avoid contamination of the fluid sample as it flows from the formation and into a downhole tool during sampling. A challenge thus lies in minimizing the contamination of the virgin fluid during fluid extraction from the formation.
FIG. 1 depicts a subsurface formation 16 penetrated by a wellbore 14. A layer of mud cake 15 lines a sidewall 17 of the wellbore 14. Due to invasion of mud filtrate into the formation during drilling, the wellbore is surrounded by a cylindrical layer known as the invaded zone 19 containing contaminated fluid 20 that may or may not be mixed with virgin fluid. Beyond the sidewall of the wellbore and surrounding contaminated fluid, virgin fluid 22 is located in the formation 16. As shown in FIG. 1, contaminates tend to be located near the wellbore wall in the invaded zone 19.
FIG. 2 shows the typical flow patterns of the formation fluid as it passes from subsurface formation 16 into a downhole tool 1. The downhole tool 1 is positioned adjacent the formation and a probe 2 is extended from the downhole tool through the mudcake 15 to the sidewall 17 of the wellbore 14. The probe 2 is placed in fluid communication with the formation 16 so that formation fluid may be passed into the downhole tool 1. Initially, as shown in FIG. 1, the invaded zone 19 surrounds the sidewall 17 and contains contamination. As fluid initially passes into the probe 2, the contaminated fluid 20 from the invaded zone 19 is drawn into the probe with the fluid thereby generating fluid unsuitable for sampling. However, as shown in FIG. 2, after a certain amount of fluid passes through the probe 2, the virgin fluid 22 breaks through and begins entering the probe. In other words, a more central portion of the fluid flowing into the probe gives way to the virgin fluid, while the remaining portion of the fluid is contaminated fluid from the invasion zone. The challenge remains in adapting to the flow of the fluid so that the virgin fluid is collected in the downhole tool during sampling.
Various methods and devices have been proposed for obtaining subsurface fluids for sampling and evaluation. For example, U.S. Pat. No. 6,230,557 to Ciglenec et al., U.S. Pat. No. 6,223,822 to Jones, U.S. Pat. No. 4,416,152 to Wilson, U.S. Pat. No. 3,611,799 to Davis and International Pat. App. Pub. No. WO 96/30628 have developed certain probes and related techniques to improve sampling. Other techniques have been developed to separate virgin fluids during sampling. For example, U.S. Pat. No. 6,301,959 to Hrametz et al. and discloses a sampling probe with two hydraulic lines to recover formation fluids from two zones in the borehole. Borehole fluids are drawn into a guard zone separate from fluids drawn into a probe zone. Despite such advances in sampling, there remains a need to develop techniques for fluid sampling to optimize the quality of the sample and efficiency of the sampling process.
In considering existing technology for the collection of subsurface fluids for sampling and evaluation, there remains a need for apparatus and methods having one or more, among others, of the following attributes: the ability to selectively collect virgin fluid apart from contaminated fluid; the ability to separate virgin fluid from contaminated fluid; the ability to optimize the quantity and/or quality of virgin fluid extracted from the formation for sampling; the ability to adjust the flow of fluid according to the sampling needs; the ability to control the sampling operation manually and/or automatically and/or on a real-time basis. To this end, the present invention seeks to optimize the sampling process.